The US military has recognized the increasing threat to its tactical aircraft from anti-aircraft infrared (IR) guided missiles. By one estimate more than 500,000 shoulder-fired surface-to-air missiles exist and are available on the worldwide market. The lethality and proliferation of IR surface-to-air missiles (SAMS) was demonstrated during the Desert Storm conflict. Approximately 80% of U.S. fixed-wing aircraft losses in Desert Storm were from ground based Iraqi defensive systems using IR SAMS. Both IR SAMS and IR air-to-air missiles have seekers with improved Counter-Countermeasures (CCM) capabilities that seriously degrade the effectiveness of current expendable decoys.
Usually, when a target platform has been detected, targeted, locked-on, and a missile fired, the in-flight missile needs to be jammed in order to avoid impact. Many IR seeking missiles require lock-on prior to launch and do not have autonomous reacquisition capability. Given an adequate hemispheric missile warning system (such as that in development), it is quite conceivable that the missile can be defeated in flight. Once the missile is detected some form of jamming technique needs to be implemented in order to cutoff the missile's chase after the target platform. One approach is to use an RF weapon (directed from the aircraft under attack, or counter-launched) to defeat the guidance electronics. For optical or IR seekers that are not “in-band” with the RF weapons, a “back-door” means of coupling the RF energy into the attacking missile must be used. Such back-door mechanisms exist; however, they are commonly considered to be unpredictable and statistically diverse, differing by orders of magnitude from missile to missile, even those of the same class, depending on the missile's maintenance history.
Directed infrared countermeasures systems [DIRCM] systems use beams of light, produced by a variety of means such as flash lamps, to exploit knowledge about the design of reticle-scan seekers to defeat their homing mechanisms or guidance systems. In many IR missiles, a reticle within the seeker (or guidance system) causes pulses of light from the target aircraft to “shine” on the missile's infrared detector. The IR detector senses the IR radiation and sends an electric signal to the guidance package, which determines the target location and allows the missile track the target aircraft's location and movement through the sky. By shining a modulated light towards the seeker, an IRCM system provides the infrared detector with extra “false” data, which deceives or “jams” the missile, causing it to miss its intended victim.
There are more than 3,000 IRCM systems deployed world-wide that protect against infrared guided threats. Despite the advantages that DIRCM systems have over flares, these systems have limitations that have prompted a move towards laser-based systems, such as the Navy's TADIRCM system and the Air Force's new LAIRCM system. LAIRCM builds upon the Northrop Grumman's widely-utilized NEMESIS DIRCM platform but replaces the flash lamp source of IR radiation with a laser source.
A laser DIRCM (LDIRCM) based protection suite against MANPADs and other IR guided threats typically includes: a Missile Warning System (MWS) and a Laser Directable Infrared Counter Measure unit, to be described bellow.
The MWS is typically an optical system in the UV or infrared wavelength range. Typically consists of a set of 4-6 imaging detectors (MWS sensor modules), each covering a sector around the platform. The imaging device is typically connected to a signal processing unit that analyzes the images received from the imagers, and decides whether the image includes the signature of a missile. It may also track the missile and provide time-dependent information about its location, may even suggest the type of the missile.
Typically, the MWS consists of a set of staring array detectors that are distributed around the platform and provide 360 coverage around the aircraft, and 10 s of degrees above and bellow the horizon. These staring array detectors usually work in the Solar Blind UV (SBUV) wavelength or the mid-infrared wavelength. There are also Doppler radar systems, which are very effective in terms of low false alarm, but provide accuracy which is typically too low for directing a DIRCM towards the missile.
The laser unit typically includes: a laser unit in Band IV wavelength range (3-5 μm), a laser unit in Band I (1.5-2.5 μm) (necessary mainly for old generation missiles. sometime produced simultaneously by the same laser as the one used for the Band IV), tracker: a thermal imaging system in Band IV, based on a cryogenically cooled detection (typically InSb or MCT detector array), and Beam-steering mechanism, for example a gimbal: a two (or more) axes system that steers the direction of the laser and the tracker towards the missile, either by steering the laser and tracker themselves, or by steering a mirror that steers the beam (the latter is usually referred to as a “mirror gimbal”).
The laser is usually an expensive part of the system. Typically laser used is a high power solid state or fiber laser, which emits a pulsed laser beam at wavelength of typically 1 μm, which is then wavelength shifted to the Band IV wavelength region using an Optical Parametric Oscillator (OPO). Such lasers are typically large, expensive and power inefficient. Thus, typically the LDIRCM is based on a single Centralized Unit, where laser radiation is emitted from a single location on the aircraft. In some extreme cases (for example, fighter jets that required protection from all directions, or of aircrafts where obscurations are too high, or large aircraft where the engines are so much apart that they require a separate LDIRCM for each wing), two systems are installed, but their structure is practically similar to the one of the Central Unit.